This is a guide about one of the most versatile and useful spells in the mage's arsenal, Spellsteal. Now, unless you have been living under a rock or in the Dalaran Sewers teaching people how to make Alcoholic drinks. Spellsteal is a spell (Brilliant deduction) that steals (again, Brillaint deduction) magical buffs from hostile enemies. The drawback is it's duration, all magical buffs have a maximum duration of 2 minutes. You can steal as many spells as you want though and in many cases it does not only help you, but also it many help the rest of your group. As they say, knowledge is power, so knowing what to steal is key when using this spell.
Some buffs many not be useful for you, but getting them removed from the caster can make things easier for the tank or the healer.
This guide will cover Wrath Dungeons and Raids.

I will split up the buffs by:
Place
Spell
Effect
Who casts it
Usefulness: X/5
Duration (if it is lower than 2 minutes)
Stacks: Y/N
Personal notes.

Without Further explanation, here is the list:

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Utgarde Keep

Bolthorn's Rune of FlameDeals 1619 to 1881 (3238 to 3762, in Heroic) Fire damage to enemies within 10 yards. Procs every 2 seconds
Casted by: Dragonflayer Runecaster
Usefulness: 4/5
Duration: 8 Seconds
Stacks: N
Note: This spell will just Aoe down mostly anything on it's path, Although it last 8 seconds which means you will have 4 ticks before it expires, it's really useful and keep the group from recieving significant damage from it.

Njord's Rune of ProtectionA magical runeshield that protects the target from 50000 (100000, in heroic) Damage for 8 seconds.
Casted by: Dragonflayer Runecaster
Usefulness: 3/5
Duration: 8 seconds
Stacks: N
Note: Although it's a shield, removing it prevents the hostile mob to be kept alive longer than intended. Note the huge amount of damage that the shield actually absorbs. Getting hold of it is better to hasten things up.

The Nexus

BloodlustIncreases the ally's attack speed by 30% for 30 seconds. (Same effect on heroic)
Casted by: Allance/Horde Berserker
Usefulness: 2/5
Duration: 30 seconds.
Stacks: N
Note: Even though there is no actual use for it, on you, stealing it might make things easier for the healer. It's upto the mage's choice if he wants to steal it or not. This haste ONLY affects Melee attacks.

Aura of Arcane HasteIncreases the casting speed of the caster by 15% and spell damage by 150 for 1 minutes (50% and 500SP(!!!), in heroic)
Casted by: Mage Hunter Ascendant
Usefulness: 5+/5
Duration: 1 minute
Stacks: N
Note: You must spell steal this buff, it does not only benefit you but also as it is an aura it benefits the whole group with it. Keep your eyes on this mob and steal his delicious buff off him.

RenewHeals the target by 60 every 3 second for 15 seconds (Same effect on heroic)
Casted by: Mage hunter Initiate
Usefulness: 3/5
Duration: 15 seconds
Stacks: N
Note: This spell stacks with Spell power, so the heal might be higher than stated above. This will just hasten things up when doing the instance.

Crystal BarkA layer of Crystal bark that will damage enemies for 200 arcane damage and has a chance to stun them from 3 seconds when struck. (same effect in heroic)
Casted by: Crystalline Keeper
Usefulness: 2/5
Duration: 2 Minutes
Stacks: N
Note: The stun itself is just the only reason you might want to remove this off him. They will recast this spell often though so it's upto the mage if he wants to steal it or not.

Mana ShieldA protective shield that absorbs 240 Damage. Takes 2 mana per point of damage. (30000 damage, 1 Mana per point of damage)
Casted by: Azure Warder
Usefulness: 0/5 (on normal) 3/5 (On heroic)
Duration: 1 Minute
Stacks: N
Note: On normal, don't even bother. On heroic, it will be better if you do steal it because you kill this guy faster.

Crystal bloomIncreases the damage done by 2% (same effect in heroic)
Casted by: Crystalline Frayer
Usefulness: 4/5
Dutation: 30 seconds
Stacks: Y, 15 stacks max. (a Grand total of 30%)
Note: This guys will cast this buff as soon as they Agroo, and plus they are everywhere. Steal a couple of their buffs and nuke away.

Azjol Nerub

Sadly, no benefitial buffs to be stolen.

Old Kingdom

Icy WindsWinds that slows your spellcasting by 50% but increases the damage done by frost spells by 75%. (Same effect in heroic)
Casted by: Frostbringer
Usefulness: 5+/5 ( for frost) 4.5/5 (for the other specs)
Duration: 2 minutes (This spell has been made to be stolen)
Stacks: N
Note: Steal this buff, and you will probably be able to fight global warming at the same time you are destroying things in this instance. This buff is INSANE for frost mages, take 2 of this buff and call me in the morning. Make sure to Dispell this if you are going to engage in single target combat though.

BloodlustIncreases the ally's attack speed by 30% for 10 seconds (same effect in heroic)
Casted by: Twilight Apostle
Usefulness: 4/5
Duration: 10 seconds
Stacks: N
Note: Hastens your spellcasting, which is a good thing by default, and slowers theirs by not having the buff. This affects all actions.

Lightning ShieldA shield of nature that causes 2 (affected by SP) nature damage when struck. Lasts 10 minutes of when the 3 charges have been used. (1600 nature damage, lasts 10 minutes and has 10 charges in heroic)
Casted by: Twilight Apostle
usefulness: 3/5
Duration: 10 minutes, due to Spellsteal it lasts 2 minutes.
Stacks: N
Note: Any damage that can be prevented is good in my book.

Drak'tharon Keep

Unholy FrenzyIncreases the ally's attack speed by 30% for 15 seconds and inflicts 232 to 268 Shadow damage every 2 seconds.
Casted by: Scourge Reanimator
Usefulness: 1/5
Duration: 15 seconds
Stacks: N
Note: I don't know why would you want to steal it, but if you do.. I suggest you dispell the buff afterwards.
This haste ONLY affects melee.

Gundrak

Earth ShieldA Shield of Nature that heals when Struck by a melee attack. Heals for 1388 to 1612 (2775 to 3225, in heroic).
Casted by: Drakkari Medicine Man
Usefulness: 3/5
Duration: 10 Minutes. 2 Minutes due to Spellsteal
Stacks: Y. 6 Charges.
Note: Remove a shield and remove the good doctor out of the equation.

Violet Hold

Arcane EmpowermentIncreases melee, movement, casting, ranged speed by 50% (same effect in heroic)
Casted by: Azure Mage Slayer
Usefulness: 3/5
Duration: 10 seconds
Stacks: N
Note: They Spawn in the portals when they are opened by Portal Guardians/Keepers. This Buff is useful, but it's duration is the big drawback of it.

Earth ShieldA shield that heals the target when struck by a melee attack. Heals by 879 to 1021 and has 20 charges. Last 2 minutes. (heals by 3238 to 3762, in heroic*)
Casted by: Erekem (And Arakkoa Windwalker in case of a wipe in heroic mode)
Usefulness: 3/5*
Duration: 2 minutes
Stacks: Y, it has 20 stacks.
Note: You can spell steal this in Normal mode Only. In heroic you can't, too bad.

Shroud of DarknessYou are covered in darkness, when damaged the attacker gains a 20% (30%, in heroic) healing debuff.
Casted by: Zuramat the Obliterator (or Void lord, in case of a wipe in heroic mode)
Usefulness: 5/5
Duration: 10 seconds
Stacks: Y, 4 times.
Note: This is the one of the few spells that are worth stealing, you will never have to stop the DPS and your healer will be greetful in the end.

Lightning Shield (II)A Shield of lightning balls, deals 1000 (3000, in heroic) nature damage. It has 10 charges.
Casted by: Sjonnir The Ironshaper
Usefulness: 5/5
Duration: 10 Minutes. 2 Minutes with Spell Steal.
Stacks: Y, 10 of them.
Note: Removing the shield will just cut 65% of all the damage this boss can do. Plus you can use it to nuke the adds when they get angry at you f